The present invention relates generally to a new apparatus and method of semi-aqueous solvent cleaning of paste processing residue from substrates. More particularly, the invention encompasses an apparatus and a method that utilizes a semi-aqueous solvent cleaning method for cleaning of paste residue from screening objects comprising the use of at least one propylene-glycol alky-ether solvent.
In the fabrication of multilayer ceramic (MLC) substrates for packaging semiconductor devices, conductive metal patterns are screened onto individual ceramic green sheets by printing, such as, extrusion printing, which uses metal mask, such as, Mo, Ni, or Cu, which are placed in contact with the green sheet, or by screen printing, which involves paste squeegee using mesh mask or emulsion mask. After screening, the green sheets are assembled and aligned, and laminated, followed by sintering operation to form multilayer ceramic substrate having internal metallurgy for mounting and interconnecting a plurality of integrated circuit semiconductor devices.
For advanced ground rule electronic packaging structures requiring to print closely spaced conductive metal patterns on a substrate, the screening masks used have highly dense fine dimension etched features. Under certain conditions such screening masks may have the problem of paste residue entrapment in the mask features in addition to a surface residue when the paste is screened to deposit conductive pattern on green sheets. This requires that the metal mask be cleaned after one or more screening passes to eliminate/minimize the possibility of defects in a subsequently screened pattern. Any defects in the screened paste pattern replicates into the final product causing yield loss.
Conductive pastes used in screening processes for delineation of wiring and/or via metallurgy, and I/O pad (input-output) patterns comprise metal particles dispersed in an organic binder and solvent vehicle along with wetting agents, dispersants/surfactants, plasticizers, and other additives, such as, thickening agents, antioxidants, and coloring agents which are all well known in the fabrication of electronic components.
Most commonly used conductive pastes in multilayer ceramic fabrication are based on molybdenum or tungsten metal powder dispersed in an organic polymer binder, such as, for example, ethyl cellulose, polymethyl methacrylate, and the like, or poly-hydrocarbon based thermoplastic resins dissolved in a high boiling solvent system.
Conductive pastes that are typically used for screening processes in multilayer ceramic comprise molybdenum, copper, tungsten, nickel, gold, tin, and the like. Because of the variety and complex chemical make-up of metal/polymer composite pastes, it is required that the mask cleaning medium and process selected be such that it provides complete and efficient cleaning of all types of pastes from screening masks and associated equipment, such as, extrusion heads or paste nozzles, etc.
Solder pastes commonly used in electronic assembly processes for interconnections constitute eutectic alloys, specifically, Pbxe2x80x94Sn, Bixe2x80x94Sn, Inxe2x80x94Sn, and related powder alloys dispersed in an organic carrier. Solder paste application onto substrates for component attachment is done by screen printing through a screen stencil, for example, stainless steel stencil, or by the dispense method. Various solder paste formulations constitute a eutectic alloy dispersed in an organic carrier, for example, a solvent, rheology modifier, fluxing agent, such as, rosin based flux. It is important to clean solder paste residue from metal mask stencils, especially in the case of ultra-fine pitch BGA (Ball Grid Array) attachment to assure defect-free printing on substrate metal pads and thus eliminate solder defects.
Traditionally used cleaning solvents for cleaning/degreasing purposes in many industrial and consumer product applications include aromatic hydrocarbons, chlorinated solvents, typically, 1,1,2-trichloroethylene, perchloroethylene, methylene chloride, and fluorochlorocarbons as CFC-113, and CFC-112. The chlorinated solvents were especially preferred because they are non-flammable, i.e., have no flash point, and are highly effective solvents.
Because of the environmental and health issues associated with the use halogenated hydrocarbon solvents in production processes, there has been major focus in the past decade to introduce environmentally safer/acceptable organic solvent alternatives, and water-based cleaning formulations. Recently, ultrasonic and spray cleaning equipment for water-based cleaning chemistry have become commercially available and several detergent compositions have been described in the patent literature for cleaning/degreasing purposes. These compositions are used, for example, for cleaning printed circuit board assemblies and other parts in device fabrication to remove soldering flux, oil/grease, and other organic residues invariably formed during bonding and assembly processes in microelectronics. These compositions are generally based on a combination of surfactants in water and/or alkaline detergent compositions, comprising alkali metal salts, such as, sodium metasilicate, sodium carbonate, tribasic sodium phosphate, sodium tripolyphosphate, and combinations thereof, as well as, highly alkaline solutions based on alkali metal salts, alkali metal hydroxides, and mixture thereof with alkanolamines. Aqueous cleaner formulations comprising this category of alkali metal carbonate and bicarbonate salts are utilized with an alkali metal silicate, as described in U.S. Pat. No. 5,234,506 (Winston), for the removal of solder flux, oils, waxes, and greasy substances, adhesives and other residues from electronic circuit assemblies, such as, printed circuit or printed wiring boards during their fabrication.
Alternate organic solvent cleaning methods are based on the use of low boiling alcohol, ketone, or ester solvents, for example, isopropyl alcohol (IPA), acetone, n-butylacetate, methyl ethyl ketone (MEK), and aromatic hydrocarbons, specifically xylene. These solvents have safety and environmental issues due to their low boiling points, high flammability, volatile emissions and VOC (Volatile Organic Compound) regulations, and thus are considered undesirable for production processes. Various aqueous cleaning alternatives designed for saponification mechanism, on the other hand, generally have a high pH ( greater than 11-13) detergent formulations. High pH aqueous cleaners have associated safety issues which require special handling and costly equipment designs to assure protection against any accidental contact exposure. With alkaline aqueous cleaners comprising alkali metal salts, for example, phosphates, silicates, hydroxides, and surface active agents, there is also an issue of wastewater management for compliance with the Clean Water Act regulations which require that all possible sources of aquatic toxicity and other environmentally undesirable constituents be removed from the effluent before discharge to surface waters. This requires special and costly wastewater treatments involving precipitation with complexing agents to form sludge that carries active ingredients of cleaning compositions, metals, and other dissolved constituents resulting in solids loading to the waste. To minimize waste and reduce cost with conservation of water in aqueous cleaning, there is currently significant industry direction toward environmentally suitable zero waste processes by devising methodology for reclamation, recycling, and reuse.
The invention is a novel method and an apparatus for semi-aqueous solvent cleaning of paste processing residue from substrates.
This invention basically is a semi-aqueous method of cleaning polymer/metal composite paste residue from screening masks/stencils and paste processing equipment in the production of multilayer ceramic substrates, organic chip carriers and cards, and of composite solder paste application in electronic circuit assembly. The method according to this invention involves a cleaning cycle using a high boiling, low vapor pressure, and substantially water soluble solvent, specifically, a propylene-glycol alkyl-ether solvent followed by a first water rinse to remove the cleaning solvent carry-over on the substrate from the cleaning tank, a second water dip/rinse (optional) to remove last traces of organic contaminants, if any, and then a final thorough water rinse and drying operation.
Therefore, one purpose of this invention is to provide an apparatus and a method that will help clean polymer/metal composite paste residue from a substrate.
Another purpose of this invention is to provide for a non-hazardous, low cost, cleaning option for paste residue from substrates.
Still another purpose of this invention is to have a semi-aqueous cleaning method for removing paste screening residue from screening equipment.
Yet another purpose of this invention is to provide a non-alkaline semi-aqueous water-based cleaning method for paste screening and paste processing equipment as a superior alternative to chlorinated solvents, volatile organic solvents and alkaline aqueous cleaning compositions in multilayer ceramic manufacturing and in microelectronic assembly processes.
Another purpose of this invention is to provide a semi-aqueous cleaning method for cleaning screens/stencils and paste processing parts by using high boiling propylene-glycol alkyl-ether solvents as a replacement of volatile organic solvents and aqueous alkaline cleaning to eliminate chemical safety concerns, volatile emissions and hazardous waste.
Yet another purpose of this invention is to provide a method for cleaning of paste residue from substrates using a high boiling organic solvent for the cleaning solvent without adding water or other additives such as surface active agents or other adjuvants such that the solvent can be easily recovered and recycled.
Still another purpose of this invention is to provide a semi-aqueous alternative to flammable solvents and aqueous alkaline cleaning compositions in cleaning screening paste residue and solder paste residue from substrates in microelectronic fabrication using a propylene-glycol alkyl-ether solvent that is effective in removing polar, non-polar, or hydrophilic and hydrophobic paste residue from metal and plastic surfaces.
It is also the purpose of this invention to provide a solvent-based semi-aqueous cleaning method for screening masks/stencils and paste processing equipment which does not require special wastewater treatment.
Yet another purpose of this invention is to provide a semi-aqueous cleaning method for paste screening masks which allows easy removal of washed-off metal by filtration and solvent recovery, for example, by distillation from the waste liquid and provides benefit of waste minimization.
Still another purpose of this invention is to provide a semi-aqueous cleaning method using a high boiling propylene-glycol alkyl-ether solvent that is compatible with metal masks, emulsion masks, electronic components and polymer adhesives used in assembly processes, and all contacting materials in the cleaning equipment.
Still yet another purpose of this invention is to provide a semi-aqueous cleaning method with an organic solvent which is non-flammable high boiling Class-III combustible, (flash point of greater than about 140xc2x0 F.), preferably Class-UB combustible, (flash point of greater than about 200xc2x0 F.), non-corrosive, and has no contact hazard concerns.
Therefore, the inventors are disclosing a first invention which comprises a semi-aqueous solvent cleaning method for cleaning of paste residue from at least one object comprising the use of at least one propylene-glycol alky-ether solvent.
The inventors are disclosing a second invention which comprises a method for cleaning paste residue from at least one object, comprising the steps of:
(a) Pre-heating propylene-glycol alkyl-ether solvent in a tank having at least one ultrasonic bath at between about 45 to about 80xc2x0 C.,
(b) Immersing said paste residue carrying object in said preheated solvent and subjecting said object to ultrasonic agitation for between about 1 to about 10 min, and
(c) Transferring said object to a tank having preheated water at between about 45 to about 70xc2x0 C., and subjecting said object to at least one immersion spray and/or ultrasonic agitation, and thereby cleaning said paste residue from said at least one object.
The inventors are disclosing a third invention which comprises an apparatus for cleaning paste residue from at least one object, comprising:
(a) at least one tank containing at least one pre-heated propylene-glycol alkyl-ether solvent in said tank, wherein temperature of said solvent in said is between about 45 to about 80xc2x0 C., and wherein said tank further having at least one ultrasonic agitator,
(b) at least one means for immersing said paste residue carrying object in said pre-heated solvent and subjecting said object to ultrasonic agitation for between about 1 to about 10 min, and
(c) at least one means for transferring said object to a tank having preheated water at between about 45 to about 70xc2x0 C., and subjecting said object to at least one immersion spray and/or ultrasonic agitation, and thereby cleaning said paste residue from said at least one object.